Many diskette recorders optically sense the index hole and hence require an opaque recording disk. Specifications for some of those recorders call for no more than 0.5% transmission at a wavelength of 940 nm. The magnetic recording layer of the recording disk of an ordinary diskette may have a thickness from 3 to 5 micrometers which inherently provides sufficient opacity to meet those specifications. However, the magnetic recording layer of typical high-density diskettes may be so thin, e.g. only 1.0 to 1.5 micrometers, as to lack sufficient opacity for optical sensing mechanisms to operate correctly.
To improve the opacity of the recording disk of a high-density diskette, one manufacturer applies a carbon black coating beneath the magnetic recording layer.
Another approach to this problem would be to incorporate a light-blocking agent such as carbon black into the film base of the recording disk. Japanese Patent Application 54-160528 filed Dec. 11, 1979 (Shiegeki et al.) reports that if the film base has a single layer, this results in surface unevenness which prevents the attainment of a suitably smooth recording layer, resulting in frequent dropouts and reduced magnetic output. The Japanese application answers this problem by a laminated film base, one layer of which contains a light-blocking agent while at least one surface layer contains no light-blocking agent. The application reports that three polyethylene terephthalate films, the center of which comprised 1% carbon black, were coextruded or laminated and then biaxially oriented and heat fixed to provide a film base for a diskette. Although the resultant 3-layer film base presumably had a smooth surface since it reportedly had good magnetic output and no dropouts, no data is given concerning either the degree of smoothness or how the magnetic output and dropouts were measured.
A sample 3-layer film base containing carbon black only in the central layer has been obtained from Toray Co., assignee of JA 54-160527. A transmission electron micrograph of the sample reveals exceedingly fine carbon black beads, the median size of which is about 30 nm. Although dispersed throughout the central layer, a significant proportion of the beads form clusters or aggregates of widely varying sizes which result in protrusions projecting from the surfaces, thus making the sample unsuitably rough for use as the film base of a high-density diskette.
JA 54-160527 suggests that the presence of carbon black in the magnetic recording layer would provide the desired opacity, but that the carbon black would displace some of the magnetizable particles and thus reduce the recording density. Not mentioned is the greater danger that the carbon might interfere with curing of the binder and consequently reduce the durability.
Japanese Patent Application 56-15301 filed Feb. 3, 1981 (Nishioka) also concerns the problem of enhancing the opacity of the recording disk of a diskette. Its answer is an aluminum vapor-deposited layer on each surface of the base film.
The magnetic recording tape of a VHS cassette should be opaque to differentiate from a transparent leader which is optically sensed to stop the drive at the end of the tape. To date, the magnetic recording layers of VHS tapes have sufficient thickness to provide the desired opacity. If future specifications should require substantially thinner magnetic recording layers, it might become necessary to modify the VHS tape to enhance its opacity.